In the field of flow meters, Coriolis flow meters are unique in that they can directly measure the mass flow rate of a fluid with little or no intrusion into the fluid stream. Because of this, they have become increasingly popular and currently account for the fastest growing segment of the overall flow meter market.
Over the last 15 years, there has been a rapid evolution of developments in the field of Coriolis flow meters. These developments have concentrated on improving performance by optimizing flow conduit shapes and introducing improved signal processing techniques and different modes of vibration.
This evolutionary process began with the introduction of the first commercially-viable Coriolis mass flow meter using a U-shaped flow conduit vibrated in its first bending mode of vibration. The signal processing scheme employed was a time delay measurement between inlet and outlet motion signals. This method could give useful results, however, it was understood at that time that the elastic modulus of the vibrating portion of the flow conduit was itself a function of temperature, and that any changes therein change the sensitivity of the device. The temperature of the flow conduit had to be measured; then, the effect of temperature upon the elastic modulus of the flow conduit had to be characterized and a compensation value added to the flow signal to minimize the effects of changes in the elastic modulus of the flow conduit.
For example, 316L stainless steel is commonly used for the flow conduit material in these devices, yielding a theoretical tensile elastic modulus vs. temperature relationship of about -2.2% per 100.degree. F. increase (in the range between 0.degree. F. and 350.degree. F.) and nearly linear for that material. Therefore, the compensation value is commonly applied in a linear relationship to account for the effects of temperature on tensile elastic modulus. It should be noted here that some meter designs depend upon the shear modulus rather than the tensile modulus, or a combination thereof, and a corresponding compensation value exists thereto.
While the prior art compensation method was simple, it was also known that 316L elastic modulus became increasingly non-linear as the temperature became colder or hotter, and in general, for most common conduit materials, the elastic modulus vs. temperature curves are non-linear. This fact therefore necessitated adding more complex temperature compensation methods to account for a wider range of materials and non-linear temperature relationships.
As more Coriolis flow meters of different designs were put into service, it was found that not only temperature, but fluid density and pressure could also effect the sensitivity of the device. This realization prompted the same type of response from manufacturers as did the temperature problem earlier described in that the effects were required to be characterized and compensated for.
In the case of density effects, many types of Coriolis flow meters can calculate the density by virtue of the natural frequency of the conduit thereby yielding a signal proportional to density that can be used to compensate for density effects on sensitivity.
In the case of pressure effects, it was found that by restricting the conduit geometry to certain design relationships, pressure effects could be minimized. In either case however, the result was either more compensation circuit complexity or geometric design restrictions.
Flow meters with straight flow conduits were later introduced into the market. These meters are subject to temperature gradients between the flow conduit and the surrounding support structure that cause stresses in the flow conduit that can alter the sensitivity and zero of the device. Several methods were therefore introduced to accommodate this added problem, such as measuring the difference in temperature between the flow conduit and its support and calculating what the stress should be and deriving a compensation value based on that difference. Methods employing strain gages have also been employed for the purpose of determining the stress level and deriving the requisite compensation value, again adding more complexity to the circuit and necessitating greater understanding of the complex relationships between stress and the change in the sensitivity of a given device.
While the prior discussion has dealt primarily with effects on the sensitivity of a flow meter, another important flow measurement parameter is the zero. Since Coriolis flow meters are highly linear devices (or are made to have linear outputs) relative to mass flow rate, the two most important mathematical factors allowing their use as flow measurement devices are therefore (a) the slope of the output signal vs. the mass flow rate therein (here defined as the "sensitivity" or "K-factor"), and (b) the value of the output signal at the intercept of the line with a zero mass flow point (herein defined as the "zero").
The zero has been a much more elusive parameter for manufacturers to control because zero shifts are not usually caused by predictable changes in material constants etc., but can be caused by a number of subtle and interrelated problems in both the mechanics of the flow conduit, and in the electronics, both by design or by imperfections therein. These zero shifts are normally encountered along with changes in fluid or ambient conditions on the device similar to those just described for sensitivity effects, e.g., changes in temperature, pressure, density, frequency, viscosity or conduit stress.
To summarize the history, as Coriolis flow meter manufacturers have discovered effects on their devices that cause errors or changes in the sensitivity of their devices, they have generally chosen to characterize, measure and compensate for each effect individually, thereby creating complex compensation methods that are more expensive and less accurate than the method disclosed herein. A similar progression has taken place toward zero effects as well.
Although these various means and methods just described (and others not described) are employed to measure, and compensate for parameters that effect Coriolis flow meter sensitivity and zero, the primary and fundamental goal of all of these have been to simply determine the sensitivity and/or zero of the device to fluid flow, and then compensate for any changes therein. What is needed in the art is a way of avoiding the need to measure and compensate. What is needed is systems and methods for directly determining sensitivity or zero characteristics, or both, of a Coriolis flow measurement device, thereby allowing overall compensation for any changes in sensitivity or zero characteristics, regardless of source.